1. Field of the Invention
The present invention relates to a microwave oven door, which can shield (or seal) the leakage of microwave. More particularly, the present invention relates to a microwave oven door having a microwave shielding structure which maximizes the shielding of the microwave leakage to outside by forming a choke structure having a short-circuiting transmission path at an end of a conductor plate, to effectively protect the human body from harmful microwaves.
2. Prior Arts
Generally, home microwave ovens are provided with a magnetron for generating microwaves. In the electric field room of a microwave oven, there is provided a magnetron for generating microwaves. Such microwaves are generated when a high voltage produced by primary and secondary induction coils of a high potential transformer that is attached on a base plate of the electric field room, is stably supplied to the magnetron, the high voltage being generated through the inductive interaction between the induction coils. Such microwaves are irradiated into a cooking chamber of a microwave oven through an irradiating tube. When the microwaves are irradiated into the cooking chamber after passing through the irradiating tube, the food placed within the cooking chamber is heated in order to cooked.
The power supply line of the magnetron mainly consists of a filament, a cathode and an anode. When the high voltage is supplied to the magnetron to generate microwaves, unnecessary microwaves radiating through cathode and filament, i.e., noises and microwaves having basic frequencies which are suitable for heating the food, are generated. Then, the noises flow back through the filament and the cathode, causing wave obstructions in nearby apparatuses.
Because such microwaves are radio waves harmful to human body, its leakage to the outside is prevented while cooking. A door of the microwave oven is provided for withdrawing the cooked food through the front of the cooking chamber. The cooked state of the food can be recognized without opening the door, and the door has a choke structure to prevent leakage of the microwaves even though it is closed.
Conventionally, doors with choke structures are divided into those of one piece-type (1 PC) and two pieces-type (2 PCS). FIG. 9 is a cross-sectional view for showing a conventional door having a 2 PCS-type choke structure, which is attached to a cooking chamber 102 of a microwave oven cavity 101. FIG. 10A is a detailed cross-sectional view for showing the 2 PCS-type choke structure as shown in FIG. 9, in which a microwave absorber is not attached, and FIG. 10B is a detailed cross-sectional view for showing the 2 PCS-type choke structure as shown in FIG. 9, in which a microwave absorber is attached. As shown in the figures, a door having the 2 PCS type choke structure comprises two components consisting of a door frame 105 to form a choke structure (or choke part) 106 and a door screen 104 having perforations. A front panel 103 is provided at the overall peripheral region of an entrance 111 of cooking chamber 102 of microwave oven cavity 101. A door screen 104 having perforations through which the cooked state of food 110 in cooking chamber 102 can be recognized from outside, is formed on the front panel 103. Front panel 103 and a sealing surface 115 formed at the peripheral portion of door screen 104 primarily seals the microwave leakage by the junction between the metal plates. At the overall peripheral region of door screen 104, a door frame 105 is integrally formed by a binding method such as a projection weld 114. Around the end portion of door frame 105, a bending portion 112 as a protruded portion having a rectangular shape, is provided. An opening 113 is provided between bending portion 112 and door screen 104 to form a choke structure 106 inside door frame 105.
As shown in FIG. 10B, a microwave absorber 107 is installed at opening 113. In FIGS. 10A and 10B, l.sub.1 and l.sub.2 mean each continuous distance between the centers of choke structure 106 consisting of opening 113 and short-circuiting wall 108, respectively. They form a microwave path.
FIGS. 11A and 11B are cross-sectional views for showing a conventional 1 PC-type door. FIG. 11A shows a conventional 1 PC-type choke structure in which a microwave absorber is not attached, and FIG. 11B shows a conventional 1 PC-type choke structure in which a microwave absorber is attached.
As illustrated in the figures, the conventional 1 PC-type door has a door frame 205 having a choke structure 206 as one PC type on a front panel 203 provided at the overall peripheral region of the entrance of a cooking chamber 202. Front panel 203 and the peripheral portion of door frame 205 meet each other (junctioned) via a sealing surface 215 in order to primarily shield the microwave leakage by the junction of metals. As in door frame 104 of 2 PCS-type shown in FIG. 9, door frame 205 has perforations to recognize the cooked state of food in cooking chamber 202, and a bending portion 212 is integrally formed at the peripheral portion of sealing surface 215 so as to have a predetermined height h which meets front panel 203. Inside bending portion 212, a choke structure 206 is provided to prevent the leakage of the microwave which has been primarily shielded by sealing surface 215. At the outer portion of choke structure 213, an opening 213 is formed, in which a microwave absorber 207 is provided. In the 1 PC-type door, the central depth l.sub.3 that corresponds to the height h of bending portion 212 forms the microwave path.
The choke structures of 2 PCS- and 1 PC-type door are all based upon the technical idea for decreasing the microwave leakage by allowing the microwave paths 109 and 209 to be 1/4.multidot..lambda..sub.0 (.lambda..sub.0 : free space wavelength of the microwave).
When the characteristic impedance of the short-circuiting transmission path (in the choke structure) is Z.sub.0, a path length (which means the length of the microwave path) is 1, and an input impedance from the transmission path entrance (which means the opening of the choke structure) to the end of the path when the end of the path is short-circuited (when short-circuited, the load impedance Z.sub.L is zero) is Z.sub.IN, a relationship Z.sub.IN =V(l)/I(l)=j.multidot.Z.sub.0 .multidot. tan .beta.l (wherein, .beta.=2.pi./.lambda..sub.0, .lambda..sub.0 : free space wavelength), is established.
The microwave leakage decreasing apparatuses (shielding apparatus) having the above 1 PC and 2 PCS type, in which the microwave path of the choke structure is determined as 1/4.multidot..lambda..sub.0, are based upon the principle of achieving the input impedance of the microwave transmission path of 1 PC type door (the distance from opening 213 to the inner surface of a short-circuiting wall 208, l), .vertline.Z.sub.IN .vertline.=Z.sub.0 .multidot.tan {(2.pi./.lambda..sub.0)(1/4.multidot..lambda..sub.0)}=.infin..
Also, as shown in FIG. 10B, the microwave transmission path of the 2 PCS-type door, the input impedance at opening 113 of choke structure 106 .vertline.Z.sub.IN .vertline.=.infin. is achieved by allowing the central continuous distance from opening 113 of choke structure 106 to the inner surface of short-circuiting wall 108, 1.sub.1 +1.sub.2 to be 1/4.multidot..lambda..sub.0,
There are several problems in the above 1 PC and 2 PCS type doors.
Firstly, the size of the choke structure is large. More particularly, the microwave transmission path is restricted to the depth of choke structure 206 in case of 1 PC type door, and therefore the size of bending portion 205 of the door should be large. In 2 PCS type door, the microwave transmission path consists of the central continuous distance of choke structure 106, 1.sub.1 +1.sub.2. Therefore, since the microwave transmission path is limited by the depth and length of choke structure 106, the choke structure of the 2 PCS type door can be formed smaller than that of 1 PC type door. However, there is still a limit in reducing the choke size while maintaining the microwave transmission path as 1/4.multidot..lambda..sub.0. Therefore, it is difficult to achieve a compact door.
Secondly, due to the insufficient microwave decreasing ability, additional microwave absorbers 107 and 207 are necessary. For example, two components of door screen 104 and door frame 105 are integrally formed by a method such as projection weld 114 to form a primary sealing surface 115 in the 2 PCS type door. During the welding process, a welding residue remains or deformation of the conductive surface due to the heat occurs on the primary sealing surface 115 so that the smoothness of door screen 104 becomes degraded. Therefore, not only the primary sealing effect is degraded but it is also difficult to form choke structure 106 having the accurate dimension via the welding process of the two conductors, and therefore, the microwave reducing ability decreases. Thus, expensive components such as microwave absorber 107 are necessary.
Thirdly, the productivity of the door is low. In the 1 PC type door, choke structure 206 is formed by only setting the depth of choke structure 206. When the microwave wavelength is considered, the choke structure that has a drawing shape of about 30 mm depth at the peripheral region of the conductor plate is very difficult to manufacture. Also, in the 2 PCS type door, two components are integrally formed via hard works such as dimension controlling, welding controlling, etc. and thus its productivity becomes very low.
Fourthly, providing the choke structure is expensive. As mentioned above, additional components such as microwave absorbers 107 and 207 are necessary and also its productivity is lowered due to the difficulties in the manufacturing process thereof, which results in high cost.
Meanwhile, a microwave ovens choke structure having a G-shaped profile is described in U.S. Pat. No. 4,645,892 (issued to Jan A. C. Gustafsson). FIG. 12A is a sectional view of an oven cavity 310 with a door 319 having the above choke structure disclosed in the above U.S. patent, and FIG. 12B is an enlarged view of the choke structure as illustrated in FIG. 12A.
At the front of a front panel 318, door 319 is provided parallel. A choke 320 is made by folding of a pre-punched sheet and has a G-shaped profile as shown in FIG. 12B. The microwave transmission path L starts from an inner portion 327, via an input opening 326, and past a partition wall 323), to a short-circuiting wall 324 and its length is 1/2.multidot..lambda. (wherein .lambda. is the wavelength) long. The microwave disappears through the U-shaped path inside the choke.
In the choke structure described in the above U.S. patent, a partition wall should be provided with transverse slots to decrease microwave leakage. Further, it is difficult to determine the size of the choke structure because the microwave transmission path leads from the inner portion 327 of the sealing surface in order to have a 1/2.multidot..lambda. length. That is, according to above U.S. Patent, the choke structure may be constructed in either a very small size or a very large size.